In unsteady flows, dune dimensions may vary considerably from fully-developed dimensions produced from a flat bed under a steady and uniform flow. Specifically, dune height and length are observed to lag discharge when the flow is non-steady, resulting in dimensions that are out of phase with the prevailing flow. This research attempts to provide some insight into the behaviour of dunes when the flow is suddenly changed, as well as the time-scale of the related dune changes. Nineteen experimental runs were carried out in the 21-m long, 0.76-m wide Sediment Transport Flume at Queen’s University. In ten of these runs the flow depth, h, was suddenly increased (h2/h1 > 1) and in nine runs the flow depth was suddenly decreased (h2/h1 < 1). In all runs, the slope of free surface was kept equal to the slope of the bed such that the change in flow rate is represented by the change in the flow depth. Seven ratios of the two flow depths, h2/h1 (varying between 0.49 and 2.29), were performed for three slopes (S = 1/792, 1/534, 1/341). The bed material was a coarse, poorly-graded silica sand (D50 = 1.0 mm). Longitudinal bed profiles were taken along the centreline of the flume approximately every 6–8 minutes to determine the transient dune dimensions and the time at which the dunes acquired their new equilibrium dimensions. This time is termed the duration of dune development, Td. Through dimensional and physical considerations, Bielenberg (2006) established that the dimensionless counterpart of Td should be a function of the material number, relative flow depth, relative flow intensity, and ratio of the flow depths h2/h1. The present experiments were carried out to investigate the influence of the relative flow intensity and h2/h1 on the duration of dune development. Results indicate that equilibrium dune dimensions do not depend on the initial shape of the bed. After the period Td, the dunes tend to be similar to those produced from a flat bed. It is found that Td is strongly dependent on h2/h1, and weakly dependent on the flow intensity. All other determining variables remaining the same, Td decreases with increasing values of flow intensity. Semi-empirical relations for the dimensionless duration of dune development are presented. / Thesis (Master, Civil Engineering) -- Queen's University, 2007-09-24 09:45:13.624

It is demonstrated that the method for locating a velocity profile origin, or plane of zero velocity, by fitting log profiles to streamwise velocity measurements is applicable to a larger range of roughness scales than previously expected. Five different sets of detailed, experimental velocity measurements were analyzed encompassing sediment-scale roughness elements, roughness caused by rigid vegetation, and large-scale roughness elements comprised of mobile bedforms. The method resulted in similar values of normalized zero-plane displacement for all roughness types considered. The ratios of zero-plane displacement, dh, to roughness height, ks, were 0.20 and 0.26 for the sediment- and vegetation-scale experiments, respectively. The results for the two experiments with bedform dominated roughness were 0.34 and 0.41. An estimate of dh/ks ranging from 0.2 to 0.4 is therefore recommended for a range of roughness types with the higher end of the range being more appropriate for the larger, bedform-scale roughness elements, and the lower end for the sediment-scale roughness elements. In addition, it is demonstrated that the location of the plane of zero velocity is temporally constant even when the bed height is not. The effects of roughness element packing density were also examined with the identification of a possible threshold at 4%, above which zero-plane displacement is independent of packing density. The findings can be applied to field velocity measurements under mobile bed conditions, facilitating the calculation of turbulence parameters such as shear velocity, by using point measurements and providing guidelines for the estimation of an appropriate value for zero-plane displacement. / Master of Science

Expériences de terrain et de laboratoire montrent que les rivières à gravier avec étendue granulométrique élevée et faible contrainte latérale, comme dans le cas des rivières à chenaux multiples, notamment les rivières en tresses, et des rivières à chenal unique avec une pleine mobilité latérale, comme les rivières à méandres, peuvent présenter simultanément variations du chenal actif à la fois dans le plan et dans la direction verticale, ainsi qu’un tri granulométrique planimétrique et vertical. En fait, les ajustements du chenal présentent une interaction complexe avec les processus de tri granulométrique: processus de transport et dépôt des mélanges de sédiments hétérogènes engendrent la formations de plusieurs motifs de tri, parmi lesquels patches qui migrent dans le chenal, notamment les sorting waves également connues sous le nom de bedload sheets. Ces sorting waves sont observées comme l’alternance des bandes de sédiments grossiers et fins dans la direction d’écoulement. Cette thèse de doctorat étudie les processus de tri granulométrique d’une distribution granulométrique bimodale à la fois dans la direction latérale et verticale avec un accent particulier sur l’étude des bedload sheets. Ce projet vise également à étudier les effets du confinement latéral sur la morphodynamique du lit de la rivière et en particulier sur les processus de tri granulométrique; et en plus, vu que le tri granulométrique joue un rôle crucial en influant sur le profil en long de la rivière, ainsi que les motifs sédimentaires et le formes du lit, étant donné le lien étroit entre la topographie du lit et la composition sédimentaire, pente, largeur du chenal actif et taux de transport des sédiments. L’analyse de ces paramètres permet de définir des feedbacks entre les variables morphologiques. Les résultats de cette étude éclairent le rôle de la contrainte latérale sur les processus de tri granulométrique et la morphodynamique du lit nu des rivières. / Field and laboratory investigations indicate that gravel bed rivers with poorly-sorted grain size distribution and low lateral confinement, such as in the case of multi-thread, braided and transitional rivers but also wandering single-thread rivers, can simultaneously present active channel variations, both in the planimetric and altimetric directions, together with planimetric and vertical sorting.Actually channel adjustments present a complex interaction with sorting processes. transport and depositional processes of heterogeneous sediment mixtures induce the formation of various sorting patterns, among which free migrating patches such as sorting waves also known as bedload sheets. Such sorting waves are mainly observed by the alternation of bands of finer and coarser material in the streamwise direction.This PhD research investigates the sorting processes of a bimodal grain size distribution both in the lateral and in the vertical direction, with particular focus on the study of bedload sheets.The main aim of this work is to investigate the effects of lateral confinement on river bed morphodynamics and in particular on grain sorting processes; and besides, given that grain sorting plays a crucial role in affecting longitudinal river profile, as well as sediment patterns and bed forms, being indeed the connection between bed topography and bed sediment composition close, we would like to provide simple theoretical analyses and tools able to predict morphodynamic bed river variations driven by sortingDifferent methodologies are applied to successfully achieve this purpose.Firstly, a deep investigation of the state of knowledge of mixed-size sediment morphodynamics, sorting processes, sorted sediment patterns and bedload sheets is carried out.Then, a 1D linear stability analysis of a uniform flow above an erodible bed composed by a bimodal mixture of sediments is performed and investigation of the associated eigenvectors, amplitude of perturbations and migrations rate is discussed. Results are presented in terms of arising instabilities dominated by sorting. Although the monodimensional shallow-water framework, the simple flow model adopted allows for the formulation of an algebraic eigenvalue problem that can be solved analytically, allowing for a deep insight into the mechanisms that drive instabilities.Then, small scale flume experiments are carried out by reproducing three different flow confinement configurations and involving a bimodal mixture of natural sediments. All the configurations show large fluctuations of parameters characterising bed system in terms of bed sediment composition, slope, active channel width and sediment transport rate. The analysis of such parameters allows defining feedbacks among morphological variables. Results of such investigations shed light on the role of lateral confinement on sorting processes and morphodynamics of barebed rivers.